The user of linear encoders is well known to measure linear position or linear-dependent data of production equipment or moving components thereof. Such encoders comprise reading head paired with a scale that encodes position. The reading head reads the scale in order to convert the encoded position into an electrical signal, which can then be decoded into position by a digital readout or motion controller. Frequently, linear encoders operate in dusty and contaminated environments and it is desirable to avoid contamination reaching both the reading head and the scale. To this end, it is known to mount both the reading head and the scale inside a sealed housing, which is affixed to a first object while allowing the reading head to be fixed to a second object.
U.S. Pat. No. 6,739,067 discloses a sealed linear encoder for determining the relative position of a first object and a second object. The sealed linear encoder includes a scale unit arranged in a hollow body that is mounted to a mounting structure associated with the first object and includes a slot which extends in a direction of measurement. A scanning unit for scanning the scale unit and a carrier connected to the second object, wherein the carrier extends through the slot. Sealing lips arranged at the hollow body and abutting the carrier so as to seal the carrier and a cover form-fitted to the hollow body for covering at least the slot.
U.S. Pat. No. 5,485,680 likewise discloses a sealed linear encoder having a pair of sealing lips disposed longitudinally for sealing a slot through which passes an entrainment member which connects a probing component with an object to be measured.
U.S. Pat. No. 8,234,792 discloses a linear encoder that includes a housing having an elongated hollow profile section including a first face end and a second face end, a cap disposed on the first face end and a seal which is disposed in sealing fashion between the hollow profile section and the cap. It appears from FIG. 1 of this patent that two pairs of sealing lips are disposed along the abutting edges of the slot along which a scanning unit slides relative to a scale inside the housing. Notably, the sealing lips are directed in the same direction such that their tips project toward the edge of the slot.
In all of these prior art arrangements, the housing is fixed to a first object and the scanning head is supported on a pedestal that penetrates through a slot in the housing and whose distal end projects outside of the slot and is anchored to a second object in known manner. The pedestal slides along the slot owing to relative linear movement between the first and second objects whereby the scanning head reads the scale.
A potential drawback with such an arrangement is that the sliding movement of the pedestal is apt to displace the sealing lips apart towards opposite end surfaces of the pedestal. This occurs because in locations remote from the pedestal, the edges of opposing sealing lips make abutting contact, but they obviously cannot abut one another on opposite sides of the pedestal because the pedestal displaces the sealing lips apart. Consequently, there are areas of discontinuity on opposite ends of the pedestal which prevent abutting contact of the sealing lips. These areas of discontinuity move with the pedestal and serve as conduits for the ingress of contamination.
This drawback is not significantly ameliorated, if at all, in those arrangements such as U.S. Pat. No. 8,234,792 where more than one pair of sealing lips are provided because at any given time the pedestal intersects both pairs of sealing lips in the same axial location so that the respective areas of discontinuity overlap on each end of the pedestal where the pedestal intersects the sealing lips.